This application is the U.S. national phase of international application PCT/FI2004/000373 filed 17 Jun. 2004 which designated the U.S. and claims benefit of FI 20030984, dated 30 Jun. 2003, the entire content of which is hereby incorporated by reference.
The present invention relates to a method of reducing the amount of fresh water required for washing pulp in the bleaching of chemical pulp. Sulfate pulp is most often bleached in a multistage bleaching plant, where the pulp is washed between the bleaching stages. Typically the washing is carried out by either pressing or by displacement washing. The present invention relates to intensifying the washing in the bleaching of sulfate pulp between the bleaching stages, when the washing is carried out using a washing device applying displacement. The displacement may be effected by means of e.g. pressure drum washers, washing presses or diffusers.
European patent publication 856079 discloses a method of intensifying the displacement washing of pulp so that at least part of the filtrate from a suction, pressing or thickening stage following the actual washing is directed to an immediately preceding washing stage as washing liquid. Also part of the filtrate from an only or a last stage of washing may be returned to the beginning of said stage to be used as washing liquid. In this way, the amount of washing liquid used for washing pulp may be increased, whereby the washing results are improved compared to a situation, where the same amount of liquid is introduced to the washing without directing the filtrate to the washing.
Finnish patent 67894 describes a washing method in connection with the bleaching of pulp, where bleaching chemical is introduced into the pulp alternately so that it displaces from the pulp the chemical used in a previous stage and displacement chemical is directed into the pulp by means of a washer. The washing is completed in e.g. a two-stage drum washer, in which the liquid displaced in the latter stage is led as displacement liquid into the first stage. Together with the displacement liquid, e.g. alkali is introduced to the first stage. This displacement-alkali treatment may be used to replace the conventional washing apparatus and alkali tower of a chlorination stage. The filtrate from the first stage may be led into a preceding washer.
A method has been developed to reduce the amount of fresh water or other clean washing liquid required for the washing of pulp in connection with bleaching. Specifically, is the method is believed to improve and intensify the washing between bleaching stages so that essentially the same purity of the pulp is reached with reduced amount of fresh water compared to prior art washing processes.
A method of treating pulp in connection with the bleaching of chemical pulp has been developed, which method comprises treating the pulp in at least an ozone, chlorine dioxide or alkali stage and washing the pulp thereafter in a washing device having an E10 -value of at least 3, preferably more than 4, whereby washing liquid is introduced in the washing device countercurrently in relation to the pulp and filtrate is removed from the washing device. The pulp is washed in the washing device so that the first washing liquid is filtrate obtained from the washing device itself and the amount of said washing liquid is 1.5-3.5 t/adt, whereafter the pulp is washed with a washing liquid introduced into the washing device from outside the device, the amount of said washing liquid being such that the dilution factor in the latter washing is lower than 1 t/adt and the total amount of washing liquid used in the washing device is such that the resulting dilution factor is over 0 t/adt.
The filtrate obtained from the washing device is fractionated to at least two flows. One fraction is preferably obtained from the final part of the filtrate, which forms less than 50%, preferably less than 30% of the filtrate. The fractionating is effected so that in the beginning the filtrates are led via one channel system off the washer and the remaining portion, being circulated inside the washer, is led via a separate channel system into the initial stage of the washing. This latter fraction is used as a filtrate received from the same washing device, with which filtrate the pulp is first washed in the washing device. After this, the pulp is further washed with clean liquid or liquids introduced from outside the washing device, which filtrate preferably is a filtrate from a bleaching stage located in the pulp flow direction later in the process. Said final fraction, which preferably comprises less than 30% of the filtrate to be removed from the process, is generated in the latter wash of the pulp and is thus cleaner than the filtrate received from the “preliminary” washing of the pulp. The amount of said fraction is 1.5-3.5, preferably 1.5-2.5 t/adt.
Thus, the washing stage relates to a single-stage countercurrent wash carried out between the bleaching stages, in which wash the dilution factor is less than 1 t/adt and part of the filtrate from that stage is taken to the beginning of said stage to be used as washing liquid. This treating of pulp with an internally circulated filtrate may be considered as a kind of preliminary washing.
Especially the invention may be applied to the washing of pulp after an ozone stage, chlorine dioxide stage and/or alkali stage. Alkali stages include e.g. E-, EO-, EP-, EOP-, P-, PO- or OP stages. Chlorine dioxide stages include e.g. (DC), C/D, D/C, D0, D1, D2 and D3 stages, with possible addition of chemicals, such as chlorine, EDTA or NaOH.
The method is preferably applicable in connection with the following washing devices: drum washers (preferably Drum Displacer® (DD)-drum washers [Andritz Oy]), diffuser washers, washing presses and suction filters having a washing efficiency of over 3, preferably over 4, typically 3-8, most typically 4-6, expressed as E10-value. A sufficiently efficient washing is inevitable in order to obtain from the latter wash a filtrate fraction, which is clean enough to be reused inside the same bleaching stage. The so-called E10 value is used to determine the washing efficiency so that washers of different types may be compared with each other. E10 value is a figure determined for a washer or a combination of several washers, which expresses how many ideal mixings said washer or combination of washers reaches. An ideal mixing, in its turn, is understood as a situation where the washing liquid is mixed into the pulp being washed so efficiently that the concentrations of both the liquid remaining in the pulp and the liquid removed therefrom are identical. FIG. 10 in an E10-value expresses the calculated consistency percentage at which the pulp exits the washer.
What is meant with dilution factor is the difference between used washing liquid and liquid exiting the washing device together with the pulp per a ton of pulp (DF=V2−L2, wherein V2 is the amount of washing liquid t/adt pulp and L2 is the amount of liquid t/adt pulp in the pulp leaving the washing device). The pulp is washed with washing liquid fed in from outside the washing device, the amount of which liquid is according to the invention such that the dilution factor in said final washing is less than 1, preferably less than 0 t/adt, most preferably less than −1 t/adt. The dilution factor of the washing device as a whole is over 0 t/adt, preferably over 1 t/adt.